It's not unusual to find 2x3's, or 2x4's turned sideways, and used as nailers for the drywall. That allows for the zero-clearance firebox to get right up to the opening. Of course, it does need to be properly constructed and you should see a header above that area for actual load bearing.

I won't get into the usual concerns about mounting over a fireplace, since these are easy enough to find here, and I assume you have already considered them.

The one major objection I have to mounting over a fireplace, as somebody that has designed and installed a lot of systems, is that you don't know what's behind the wall... but you've got that covered.

As long as it looks solid, I wouldn't hesitate to mount on the wall. I don't know if it's drywall or has a brick facing - if brick, drill though the mortar joints to mount so that is easily repairable should you want to take it down at some point.

To be more specific I took some pictures. I have about a 1.5' space between the stud and the flue going up from the fire box. @yosemitebill do you think i'll need to reinforce those? I see the header and the span of the wall is flanked by 2x3 that are not turned sideways. Between the outside 2x3's are three 2x3's turned on their side.

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Actually, from the pics it looks like 2x4's on their sides, but that's neither here nor there.

If you're concerned about some bowing, you could always run a horizontal stringer along the backside of the studs near the top of the mount to spread the load.

As far as cutting in for an AC outlet, and the low voltage wiring (A/V connections) you've got it made in the shade!

what about heat for the cables running behind the wall? i'm assuming these metal fire boxes are meant to keep heat in? otherwise my house would be on fire. Just wondering if the ambient heat can cause problems for the cables. I was thinking taking a pvc pipe and wrapping it up in thermal wrap to keep it nice and insulated from heat.

and by horizontal stringer you mean taking a 2x3 or 2x4 and running it behind the 2x3's on their side and sandwiching it between the 2 outer 2x3's at are not on their sides?

From your description and what's visible in the pics, this is a zero clearance fireplace box that is designed to be placed against combustible materials i.e. the studs and wall - it won't get hot enough to be of concern regarding the A/V wiring.

With the access you have I wouldn't worry about PVC conduit unless this access is going to be blocked off. Then it is there to allow for easier updates to your in-wall A/V wiring. You do, however, still need to install an AC outlet on the wall behind the television.

Yes, by a horizontal stringer, I simply meant to add some additional support across the studs if you feel it is necessary.

awesome..you've been a great help..appreciate the time and advice. I may be back with more questions as I move forward hah